Thorough Comparison of Position Correction Information: Differences Between Network RTK and Satellite CLAS/MADOCA and a Guide to Introducing LRTK

Table of Contents

• What is position correction information? Its necessity and meaning in RTK surveying

• Mechanism, advantages, and limitations of network RTK

• Mechanism, advantages, and limitations of satellite augmentation (CLAS / MADOCA)

• Comparison of network RTK and satellite augmentation methods (accuracy, latency, stability)

• Criteria for choosing by region, application, and communication environment

• Practical usage examples (civil surveying, structural construction, mountainous areas, disaster sites)

• Benefits of introducing a simple RTK surveying system that supports both methods with LRTK

• FAQ

What is position correction information? Its necessity and meaning in RTK surveying

Position correction information refers to the data used to correct positioning errors of GNSS (Global Navigation Satellite Systems such as GPS) for high-precision surveying. Standalone GNSS positioning typically incurs errors of several to a dozen meters due to satellite orbit and clock errors and ionospheric and tropospheric delays. While such errors are acceptable for everyday car navigation or smartphone maps, civil engineering surveying and construction sites often cannot tolerate errors of even several centimeters. To cancel out positioning errors, high-precision positioning techniques like RTK use correction data from reference stations or error correction information broadcast from satellites to improve accuracy. This correction data is the so-called “position correction information,” and when applied in real time it can improve GNSS positioning accuracy to the centimeter level.

In RTK surveying (Real Time Kinematic), two GNSS receivers are used: a reference station with known coordinates and a rover that observes while moving. The difference between the reference station’s measured positioning result and its true position is computed and sent to the rover as correction information via communication. The rover applies that correction to its own observations in real time, canceling out atmospheric and satellite errors and enabling centimeter-level accurate positions. Thus, position correction information is the key to RTK surveying, and only by properly obtaining and using correction information can high-precision GNSS positioning be realized on-site.

Mechanism, advantages, and limitations of network RTK

Network RTK refers to methods that achieve centimeter-level positioning by transmitting correction information from a base station to a rover in real time. Typical examples are local RTK, which installs a reference station near the site and sends correction data via radio (e.g., UHF), and network RTK (such as VRS), which uses data distributed over the Internet from a network of reference stations. Both use the basic principle of delivering correction information from ground reference stations to the rover via communication.

The greatest advantage of network RTK is its extremely high accuracy and immediacy. Real-time relative positioning can remove most satellite positioning error factors, and under good conditions horizontal accuracy of ±1–2 cm (±0.4–0.8 in) and vertical accuracy of ±3–4 cm (±1.2–1.6 in) can be achieved. Initialization time to obtain a fixed solution (resolving from float) is short—typically a few seconds to about 30 seconds—and once a fixed solution is achieved, high-precision positions can be obtained continuously in real time. Network RTK is suited to tasks requiring immediate high-precision coordinates on site, such as machine guidance and stakeout. For local RTK, proximity between the reference point and survey points makes it easier to maintain consistency with reference coordinates, while network RTK can secure stable accuracy over wide areas using virtual reference point data from a distributed network of continuously operating reference stations (e.g., GEONET by the Geospatial Information Authority of Japan).

However, network RTK has several constraints. First is dependency on communication environment. Radio-based systems are limited to the radio coverage range from the base to the rover (typically a few km), and face terrain obstructions and distance limitations. Network RTK requires mobile or Internet connectivity, and correction data cannot be received in tunnels or mountainous areas outside communication coverage. If communication is lost, positioning accuracy immediately degrades, and prolonged loss of corrections will prevent maintaining a fixed solution. Second, a reference station must be provided. Setting up your own reference station requires tripods, antennas, radios, and setup effort, and stationary high-precision receivers are expensive. Using a network service also usually requires subscription or contract with private or public services, possibly incurring monthly fees (e.g., VRS distribution services). Furthermore, RTK using a single reference station suffers degradation of correction accuracy as baseline length (distance from the station) increases, making it unsuitable for very wide-area positioning beyond several tens of km unless switching to another base station is performed. In summary, network RTK offers high accuracy and immediacy, but is highly dependent on communication infrastructure and reference stations.

Mechanism, advantages, and limitations of satellite augmentation (CLAS / MADOCA)

Satellite augmentation methods improve positioning accuracy using augmentation signals from artificial satellites without relying on ground communications. Representative services are Japan’s QZSS (Quasi-Zenith Satellite System, “Michibiki”) centimeter-level augmentation CLAS (Centimeter Level Augmentation Service) and the wide-area PPP service MADOCA-PPP (Multi-GNSS Advanced Orbit and Clock Augmentation) that uses QZSS. These services broadcast correction information via the satellites themselves, so if a receiver can pick up the supported signals, correction information can be used without ground reference stations or communication lines.

CLAS is a PPP-RTK type augmentation service provided for use in Japan. It models satellite orbit and clock errors and ionospheric and tropospheric delays in an SSR (State Space Representation) format based on observation data collected from the Geospatial Information Authority’s network of continuously operating reference stations (about 1,300 stations) and broadcasts these corrections from QZSS satellites. CLAS corrections are transmitted nationwide on QZSS L6-band signals (L6D/QZS-L6), and compatible receivers can obtain centimeter-level positioning directly from satellites, even in mountainous areas without mobile coverage. Because users do not need to set up reference stations themselves, CLAS provides generally uniform accuracy across Japan. CLAS positioning, also called PPP-RTK, requires about 1 minute of initial convergence, and once stable it reaches approximately 5–7 cm (2.0–2.8 in) horizontal accuracy (RMS). Although slightly less accurate than local RTK, CLAS’s ease of use without special communications infrastructure has promoted its adoption across surveying, agricultural machinery, and drone positioning in Japan.

MADOCA-PPP is another QZSS-based satellite augmentation that offers wide-area PPP covering as far as Asia and Oceania. It estimates satellite orbit and clock errors from data collected by global networks of reference stations (IGS, MIRAI, etc.) and broadcasts those corrections on the L6 band (L6E) as well. Like CLAS, it is usable without local reference stations, but being PPP it requires a longer initial convergence time—often 20–30 minutes—and yields post-convergence accuracy of around 10 cm horizontally (and sometimes worse) compared to RTK or CLAS. MADOCA’s advantage is its large coverage area, making it usable offshore, on remote islands, and abroad where terrestrial communication infrastructure is unavailable. In addition to correction value utility, the precise clock and orbit information provided via satellite has applications in improving weather prediction (estimating atmospheric water vapor) and time synchronization technologies. MADOCA began full operation in 2024 and currently prioritizes wide-area coverage over immediate real-time responsiveness.

The main advantage of satellite augmentation is that it does not depend on ground infrastructure and provides wide-area correction with ease. With a CLAS-capable receiver, centimeter-level positioning can be performed anywhere in Japan (where the sky is visible) without additional cost. Even in remote mountain areas or immediately after a disaster when communications are disrupted, QZSS signals can enable high-precision GNSS positioning if they reach the site. Since correction information is broadcast free of charge from national satellite systems, there are no monthly running costs. Also, performance does not degrade when many users access the service simultaneously because the corrections are broadcast from satellite to all users at once, giving high scalability.

However, satellite augmentation also has constraints. First, compatible receivers are required. Receivers and antennas that can receive QZSS L6 signals are necessary; conventional L1/L2-only devices or typical positioning modules cannot receive CLAS/MADOCA. Next, initial convergence takes time: CLAS typically requires several tens of seconds to about 1 minute, and MADOCA may need up to about 30 minutes before reaching centimeter-level accuracy. Unlike RTK, it lacks RTK-like immediacy and may be unsuitable for tasks demanding instant responses. The achievable positioning accuracy is also slightly lower than RTK: CLAS and MADOCA horizontal accuracy is generally within about 10 cm, which leaves more margin of error compared to RTK’s few-centimeter performance. Vertical accuracy in particular tends to be worse with satellite augmentation, making it less suitable for precise height surveys. Moreover, reliance on satellite signals makes performance sensitive to reception conditions: open sky is fine, but in forests or urban canyons CLAS’s L6 signals may be blocked and corrections lost. Update intervals for correction data are also coarser than RTK’s per-second updates (typically updates every few to several tens of seconds), so RTK real-time corrections better follow rapid error variations. Thus, satellite augmentation is a method that offers “no-ground-infrastructure convenience and wide applicability, but it depends on environment and use case.”

Comparison of network RTK and satellite augmentation methods (accuracy, latency, stability)

Based on the above, let us compare network RTK and satellite augmentation (CLAS/MADOCA) in terms of accuracy (positioning error), latency (initial convergence and real-time responsiveness), and stability (ease of continuous use). The table below summarizes the characteristics of both.

As the table shows, network RTK is locally confined and communication-dependent but superior in accuracy and immediacy. Satellite augmentation sacrifices some accuracy and has time lag but offers the advantage of being usable across wide areas without reliance on communication infrastructure. Stability depends on conditions: network RTK provides continuous immediate correction as long as radio or network is maintained, making it strong for dynamic positioning and quick return to fixed solutions after short losses of satellite lock (if a sufficient number of satellites remain). Satellite augmentation, while usable anywhere without communications, cannot perform where augmentation signals are blocked, and re-convergence after interruption can take time. Conversely, in open-sky environments for static continuous observation, satellite augmentation can stably maintain centimeter-level accuracy. Ultimately, the key decision is whether to prioritize “accuracy and responsiveness” or “convenience and wide-area applicability.”

Criteria for choosing by region, application, and communication environment

Which method to use depends on the site conditions and the required accuracy. Below are practical criteria for selecting the most suitable method based on regional characteristics, application, and communication environment.

• Communication infrastructure availability: Whether mobile or Internet connectivity is available at the site is a major criterion. In urban areas or sites with sufficient communication infrastructure, network RTK (network-based RTK) enables quick, high-precision positioning. In contrast, in mountainous areas, remote islands, or offshore locations without mobile coverage, network RTK is essentially unusable, making satellite augmentation (especially CLAS) the only option. In communication blackspots, CLAS/MADOCA can be the lifeline for high-precision positioning.

• Required accuracy and immediacy: If you need near-millimeter precision or centimeter accuracy immediately after powering on, network RTK is appropriate. For precise installation or machine control requiring within ±2 cm, RTK’s benefits are significant. If 10 cm-level error is acceptable or a minute or two of initial convergence is tolerable for surveys or monitoring, satellite augmentation is practically sufficient. If you prioritize wide-area coverage and convenience over immediate response, CLAS/MADOCA are strong candidates.

• Survey coverage area: If the work area is small and always within a few km of a reference station, network RTK is fine. For very wide-area surveys (e.g., tens of km of road or mobile measurements), a single base station cannot cover the whole area without accuracy loss; consider satellite augmentation or nationwide network services. In Japan, CLAS-capable equipment can provide consistent positioning nationwide, and for cross-border or very wide-area surveys, MADOCA or other countries’ SBAS/PPP services can be considered.

• Work environment (obstruction conditions): In forests or urban canyon environments with poor sky visibility, GNSS may fail. Both methods struggle, but in terms of recovery speed after temporary obstruction, network RTK is advantageous: in wooded areas you may have to wait for convergence with CLAS each time, whereas RTK can return to a fixed solution quickly if corrections are continuously received. In largely obstructed sites, satellite augmentation also loses effectiveness because augmentation satellites can be blocked. Thus, as a rule of thumb, satellite methods are preferred in open-sky locations, while network RTK may be better when obstructions cause intermittent reception.

• Equipment and cost: Initial acquisition and operating costs are also important. If your organization already has RTK base stations and network contracts and professional surveying equipment, continue using network RTK. If you want to introduce high-precision positioning easily without such infrastructure, CLAS-capable GNSS receivers are attractive: the satellite augmentation signals are free, lowering running costs. However, you must consider purchase costs of compatible receivers (high-precision GNSS receivers require investment in any case, though CLAS-capable models have become more affordable).

In summary, the general guideline is: “Use network RTK where communication is available and the highest accuracy is required; use satellite augmentation where communication is difficult or ease of use is a priority.” Recently, receivers supporting both methods in a single unit have appeared, enabling seamless switching between network RTK and satellite augmentation according to site conditions.

Practical usage examples (civil surveying, structural construction, mountainous areas, disaster sites)

Here are typical scenarios showing how network RTK and satellite augmentation are used in practice.

• Civil engineering surveying (typical earthworks and land surveying): In relatively open sites with limited distance from reference points, network RTK is the mainstay. With a base station established at a known point, high-precision real-time surveying is possible in the surrounding few kilometers. With the spread of network RTK services, especially near urban areas, VRS corrections via mobile networks enable single-person surveying. For wide-area topographic surveys, GNSS surveying can be more efficient than total station surveys. For large earthworks that include mountainous or communication-blackout sections, combining CLAS-capable equipment to switch to satellite augmentation in no-coverage areas is an option.

• Structural construction (precise layout and construction management): For bridges, plants, or other structure installations that require near-millimeter control and immediate position verification, network RTK’s immediacy and high accuracy are indispensable. It is common to mount rovers on construction machinery or prisms and set up a mobile reference station on-site for real-time guidance. Network RTK allows for on-the-spot verification of blade positions or bolt locations, speeding up as-built control. If the site is in a mountainous area or inside tunnels where base station radio cannot reach, satellite augmentation (CLAS) can be used to locate machinery or carry coordinates from outside the tunnel into the interior. Generally, precise construction management favors network RTK, but satellite augmentation can be used as a supplementary method for flexible site response.

• Mountainous area surveying (infrastructure inspection, forestry, topographic surveys): In remote mountain areas with limited mobile coverage, CLAS-capable GNSS receivers are effective. For tasks such as forest road surveys or mountain infrastructure inspections, operators can set up a receiver on-site and after a few minutes obtain position within 10 cm using satellite augmentation without erecting a base station. Activities that previously required installing triangulation points on peaks can be done with CLAS, greatly improving efficiency. Note, however, that dense forest can block satellite reception, so GNSS viability must be assessed regardless of augmentation method. Practically, using network RTK in mountain areas typically requires setting up your own radio base station each time, so CLAS is often the more pragmatic option.

• Disaster site use (damage assessment and recovery planning): After large disasters, rapid measurement of terrain change and damage extent is required, but communications may be down. Satellite augmentation is extremely useful in such situations. For example, surveying teams entering an earthquake-affected area can immediately start surveying with CLAS-capable GNSS receivers and tablets without spending time installing bases. Coordinates of landslides or breached levees can be recorded on-site to inform recovery plans. While detailed recovery construction will require the precision of network RTK, CLAS’s simplicity and no-communication requirement make it ideal for initial damage mapping and rough surveys. The ability to position even before communications are restored is invaluable in disaster response.

In practice, it is important to intelligently use network RTK and satellite augmentation according to conditions. Hybrid operation is increasingly realistic: using network RTK as the primary method while switching to CLAS in areas without communication is a common flexible approach.

Benefits of introducing a simple RTK surveying system that supports both methods with LRTK

Finally, let us introduce LRTK, a modern solution that supports both network RTK and satellite augmentation. LRTK combines a smartphone with a compact high-precision GNSS receiver to achieve centimeter-level positioning, and it is attracting attention as a simple RTK surveying tool that beginners without specialized knowledge can operate. Where RTK once required expensive fixed equipment and complex configuration, LRTK enables start-up by attaching a palm-sized receiver (LRTK Phone) to a smartphone and launching an app. There is no need for complex base station setup or radio adjustments on site; a single tap can start centimeter-accurate position measurement.

A major feature of LRTK is that it supports both network RTK and satellite augmentation. It can receive network RTK corrections (Ntrip distribution) via a smartphone’s Internet connection and can also directly receive QZSS CLAS signals. Thus, one device can use network-based RTK in urban or office-adjacent environments and automatically switch to satellite augmentation in mountainous or out-of-coverage locations. This adaptability enables stable centimeter-level positioning virtually anywhere. LRTK receivers are typically triple-frequency and use raw multi-GNSS data to achieve professional-level accuracy comparable to high-end GNSS: planar position ±1–2 cm (±0.4–0.8 in) and height ±3–4 cm (±1.2–1.6 in). CLAS support allows continued positioning even where mobile signals are unavailable, enabling tasks like surveying by vehicle in the mountains or solo surveys on remote islands.

Introducing LRTK yields many benefits. First, it improves survey efficiency: tasks previously requiring two people can be done by one, alleviating labor shortages. The small, lightweight equipment is easy to carry, enabling rapid increase of survey points even in rough terrain. Second, it reduces cost: reliance on expensive dedicated equipment or base station networks is reduced, making centimeter-level positioning accessible to small firms. Third, it enhances data utilization: LRTK integrates smartphone apps with cloud services, enabling immediate sharing and analysis of field-acquired position data, supporting DX (digital transformation) from surveying through design and construction management.

In particular, hybrid support for both network and satellite methods means LRTK can implement the “right tool for the right place” selection without requiring the user to decide. The system continuously acquires the optimal correction method, letting operators focus on measuring points rather than choosing correction sources. For example, flat construction sites maintain network RTK corrections for high accuracy, and when entering shady mountain areas the system automatically switches to CLAS reception. As a result, LRTK ensures stable high-precision positioning across environments, dramatically improving productivity and reliability in surveying and construction. By delivering top performance without needing users to understand correction differences, LRTK is poised to become a powerful solution for future civil surveying sites.

FAQ

Q: Which is more accurate, RTK or satellite augmentation (CLAS)? A: Generally, RTK using a single reference station or network RTK is slightly more accurate than CLAS. Network RTK typically achieves horizontal errors around 2–3 cm (0.8–1.2 in) under ideal conditions, while CLAS, being a satellite-based model correction, is typically around 5–10 cm (2.0–3.9 in) on average (vertical accuracy especially favors RTK). However, CLAS is practically sufficiently accurate for many surveying and agricultural applications. In short, choose RTK for the highest accuracy and CLAS if you want ease of use with adequate practical accuracy.

Q: Is there a fee to use CLAS or MADOCA correction information? A: No. CLAS and MADOCA-PPP correction signals are provided by the government and are free to use. With a compatible GNSS receiver, you can receive the corrections from satellites at no charge. Note that network RTK services may require separate subscriptions or fees, and receiver purchase costs for CLAS/MADOCA compatibility apply—but those are equipment costs, not service fees. In terms of running costs, CLAS/MADOCA are economical because they do not require subscription or communication fees.

Q: What specifically differs between CLAS and MADOCA, and how should I choose? A: Both are QZSS-based satellite augmentation services, but they differ in coverage and method. CLAS is designed for Japan with centimeter-level augmentation and about 1 minute of initial convergence to achieve a few-centimeter accuracy; it uses domestic reference station data and is effective for real-time applications like agricultural machinery and surveying. MADOCA-PPP targets the Asia–Oceania region as a wide-area PPP service; it typically needs about 20 minutes of convergence and yields roughly 10 cm accuracy. MADOCA emphasizes universal usability across wide regions (including overseas and marine environments), prioritizing “one device works anywhere” over immediacy. For domestic work, CLAS is generally preferable for faster convergence and higher accuracy; use MADOCA where CLAS cannot be received or for wide-area/overseas operation. Devices that support both can switch automatically.

Q: Can LRTK really provide centimeter-level positioning even without mobile coverage? A: Yes. LRTK supports the QZSS CLAS signal, so even where mobile networks are unavailable, the receiver can obtain correction information directly from satellites and maintain centimeter-level positioning. For example, in a mountain area where a smartphone loses mobile signal, an LRTK receiver can continue receiving corrections from the overhead Michibiki satellites, enabling high-precision measurements. When mobile coverage returns, it can automatically switch to network RTK corrections, maintaining the best available method without user intervention. In short, implementing LRTK enables high-precision positioning regardless of the communication environment, greatly expanding the range of surveyable sites and proving valuable in mountainous and disaster-affected areas.